Foam producers use aromatic polyester polyols along with other ingredients to produce polyurethane foams. Other ingredients may include polyether polyols, silicone surfactants, various amine or metal catalysts and blowing agents. This type of blend is called “system blend” or “polyol blend.” The polyol blend is one of two important components for making polyurethane foam. The other important component is isocyanate. Another important ingredient is a blowing agent. Without a blowing agent, it is impossible to make low-density cellular plastics with very high strength to weight ratio. Suitable blowing agents are hydro fluorocarbons (HFC), hydrocarbons, especially the pentane isomers, and water (carbon dioxide).
Water reacts with isocyanate to form an unstable carbamic acid, which decomposes to form an amine and carbon dioxide. The amine immediately reacts with additional isocyanate to form a substituted urea. The potential problems of employing water as a blowing agent are numerous. Urea would cause polyurethane foam to be brittle and friable, which deteriorates the adhesion of foam to a substrate. Carbon dioxide has higher permeability than air through the polyurethane cell window that could cause low-density polyurethane foam to shrink. One mole of water would react with two moles of isocyanate and release one mole of carbon dioxide. It is not a very economical way of obtaining blowing agents since isocyanate is an expensive component. Carbon dioxide is a gas with higher thermal conductivity than hydrofluorocarbons (HFC) and hydrocarbons. Water can not reduce the viscosity of “polyol blend” since it generally is not very soluble in most common polyethers or polyesters.
The most common HFCs in North America are: HFC245fa and HFC134a. Both are popular physical blowing agents. The disadvantages of employing HFCs are varied. Because they have a boiling point below room temp, they need a pressure rated vessel to hold the “polyol blend,” special caution would be needed when handling the “polyol blend” that contains HFC245fa in hot weather since its boiling point is 15.3 degrees C. Water is needed to replace some of the HFC245fa in the “polyol blend” to alleviate the vapor pressure in drums. However, some the physical properties of foam would be compromised if too much water is used in the “polyol blend.” Compared to water and hydrocarbons, HFC245fa is an expensive blowing agent.
As to hydrocarbons, the most popular are isomers of pentane. In the PUR/PIR foams markets, there are three isomers of pentanes being used. Lamination employs a blend of normal pentane and isopentane. The ratio varies from 0.2/0.8 to 0.8/0.2. System houses use 95 percent cyclo pentane and blends of cyclo/iso pentane in ratios varying from 20/80 to 50/50 for both spray application and pour in place (PIP) systems.
The advantages of employing cyclo pentane include the following. It is cheap and abundant, and has a boiling point of 49° C. There is no problem due to vapor pressure of “polyol blend” that contains cyclo pentane or cyclo/iso blends in the containers. Also, the thermal conductivity of cyclopetane is better than that of carbon dioxide.
The disadvantages of employing cyclo pentane include the following. It is a flammable liquid, and special handling is needed. It is not very compatible with most current aromatic polyester polyols, which have high hydroxyl number and high functionality. It reduces the water solubility of “polyol blend”.
When comparing all three blowing agents, the cyclo pentane and cyclo/iso pentane blends are the strongest candidate for polyurethane system applications.
Reacting a natural oil with polyethylene terephthalate in a solution at about 450° F. has been discussed in U.S. Pat. No. 6,133,329, which is incorporated by reference in its entirety herein. Pentane solubility can be increased by using hydrophobes and high molecular weight glycols in the polyester polyol formulations. However, as is shown by Example 5 of U.S. Pat. No. 6,133,329, the improvement in pentane solubility was marginal. The composition of the polyol is listed below.
Weight %Ingredients25.95Diethylene Glycol12.44Triethylene Glycol5.7Tetraethylene Glycol30.7Terephthalate4.68Pentaerythritol20.54Corn Oil
The physical properties and pentane solubilities are listed below.
Hydroxyl number220Viscosity at 25 degrees C.3,500cpsPercent normal pentane solubility15.25Percent iso pentane solubility16.67Percent cyclo pentane solubility20
Although there is an improvement in pentane solubility by combining high molecular weight glycols and corn oil, the benefit comes at very high cost because heavy glycols are much more expensive than corn oil. Polyester polyols made this way are not commercially feasible in today's market.
As discussed in U.S. Pat. No. 6,133,329, pentane solubility can also be increased by reducing the hydroxyl number of polyol. However, adding an additional 160 OH NO polyol showed limited improvement. The compositions of 160-hydroxyl number polyol are as follows.
Weight %Ingredients12.38Diethylene Glycol22.24Triethylene Glycol17.12Tetraethylene Glycol27.76Terephthalate20.5Corn Oil
The physical properties and pentane solubilities are listed below.
Hydroxyl number160Viscosity @ 25 degrees C.1,800cpsPercent cyclo pentane solubility19.5
Commercially available polyester polyols have some pentane solubility, but higher solubility is needed. The commercially available polyols have 5 to 20 percent cyclo pentane solubility. This solubility is not sufficient to make commercially acceptable low density polyurethane foam because as more pentane is added to the polyols, pentane separation occurs. To combat pentane separation, emulsifiers may be added, but emulsifiers lack long term stability and are dependent on many factors. A polyester polyol for low density polyurethane foam applications with at least 25 percent pentane solubility is desirable because it will yield a clear and soluble “polyol blend.” Also, tailored reaction conditions are needed to provide different levels of pentane solubility with the addition of a comparable amount of hydrophobe to a polyester polyol.